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1 # NBS I 1 I I 1 II 1 II 1 nil nil nil III II nil mil iii i n i i^PUBLiCATIONS A111D7 3^1033 ) NBSIR 84-2937

Validation of Daylight Prediction

With CEL-1

S. Treado

C. Francisco

D. Holland

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

December 1984

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OF STANDARDS

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VALIDATION OF DAYLIGHT PREDICTION

WITH CEL-1

NBSIR 84-2937

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S. Treado

C. Francisco

D. Holland

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

Decennber 1984

Prepared for: Naval Civil Engineering Laboratory

Port Hueneme, CA 93043

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

FOREWORD

This report documents the results of National Bureau of Stsuidards (NBS)

research in support of the Naval Civil Engineering Laboratory. The

report summarizes work conducted during the period January 1983 through

January 1984.

We wish to acknowledge the helpful interest and guidance of the

sponsor's Project Officer, Mr. Willieim Pierpoint.

DISCLAIMER

Certain commercial equipment, instruments, or materials are identified

in this report to specify the experimental procedure. Such

Identification does not imply recommendation or endorsement by the

National Bureau of Standards nor does it imply that the materials or

equipment Identified are necessarily the best available for the purpose.

ACKNOWLEDGMENTS

The authors wish to acknowledge the efforts of all those who

participated in this project and who provided insightful comments during

the review process.
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ABSTRACT

Calculations of Interior illuminance levels using the CEL-1 computer

program are compared to measurements, for a typical office space. The

comparisons are made for a wide range of sky conditions, solar

intensities and seasonal intervals using a north facing window. The

statistical uncertainty associated with the interior daylight

calculations is examined and the sources of the uncertainty are

discussed. The results show that for all sky types, the interior

Illuminance calculations have an uncertainty of +/-24JJ, with the major

uncertainties being due to unique and unpredictable sky luminance

distributions and difficulty in determining the appropriate sky

condition from standard weather data.

Keywords: building energy, daylight, illuminance, fenestration, solar

radiation, window

lii
TABLE OF CONTENTS

Pflgg

ABSTRACT ill

LIST OF TABLES v LIST OF FIGURES vi

1. INTRODUCTION 1

2. THE CEL-1 LIGHTING COMPUTER PROGRAM 2

3. DESCRIPTION OF THE MEASUREMENTS 4

4. RESULTS 5

5. CONCLUSIONS 7

6. REFERENCES 8

Iv
LIST OF TABLES

Table 1 . CEL-1 Input File Listing 3

Table 2. Test Room Dimensions and Reflectances 4

V
LIST OF FIGURES

Figure 1 . Layout of Test Room 9

Figure 2. Graph of CEL-1 Results vs Measured Data For Julian

Date 91 10

Figure 3* Graph of CEL-1 Results vs Measured Data For Julian

Date 1i»9 11

Figure 4. Graph of CEL-1 Results vs Measured Data For Julian

Date 189 12

Figure 5. Graph of CEL-1 Results vs Measured Data For Julian

Date 349 13

Figure 6. Graph of CEL-1 Results vs Measured Data For Julian

Date 355 14

Figure 7. Scatter Plot of Measured Data vs CEL-1 Calculated

Results 15

Figure 8. Effect of Sky Luminance Variations on Interior

Illumination 16

vl
1 . INTRODUCTION

Electrical energy for Interior lighting is a significant portion of

building total energy requirements, and also a large component of the

cooling load [1,2,3]. Many studies have shovm that the effective use of fenestration to provide daylight for interior spaces can be beneficial in reducing annual total building energy requirements by reducing

both lighting and cooling energy use [4,5,6,7f8],

When a building is being designed, the solar, thermal and daylighting

impacts of various fenestration designs must be evaluated to determine

the net energy Impact of each design. In this manner, the optimum

design may be determined and specified. The evaluation can consist of

the application of various design procedures and guidelines, ranging

from hand calculation procedures, to design nomographs, to detailed

computer simulations. One of the critical aspects of any evaluation

of fenestration performance is the prediction of Interior illuminance

levels due to daylight. Such a prediction is needed in order to

determine the level of electric lighting required to maintain the

desired Interior illuminance. Additional calculations are required to

determine the electric power required by the lighting system to

provide the difference between the daylight illumination and the

minimum desired illuminance level.

The purpose of this report is to validate the interior daylight

prediction portion of the CEL-1 Lighting Computer Program. This

program was developed by the Naval Civil Engineering Laboratory and

possesses the capabilities to calculate interior illuminance for all

sky types and to model actual lighting systems, including luminaires

and dimming controls. Compau*ison data for the validation were obtained

from measurements at the NBS Daylight Research Facility, using a fullsize test room with a north-facing, unobstructed window. The test

room was modeled and daylight levels calculated using CEL-1 and the

results compared to the measured illuminance levels for a variety of

sky conditions.

1
2. THE CEL-1 LIGHTING COMPUTER PROGRAM

The test room was modeled using the CEL-1 computer simulation

procedure. Details of developing a CEL-1 Input file and executing the

program can be found In other publications [9*10]. Basically, the

test room layout, dimensions, and surface reflectances are modeled,

and weather data is used to generate a model of sky and solar

lllumlnace. Flux exchange calculations then are used, after each room

surface Is subdivided Into small surfaces, to determine Interior

Illuminance at the point of Interest.

In Its normal mode of operation, CEL-1 reads solar radiation data,

direct and diffuse, from a weather tape. This Irradlance data Is

treinslated Into Illuminance data which, along with the current solar

angles, are required for the Interior illuminance calculations. For

the validation, the measured irradlance levels were used to drive the

CEL-1 predictions, so that the predictions could be compared to the

measured Illuminance levels.

The CEL-1 input file listing is given in Table 1

2
Table 1

CEL-1 Input File Listing

ROOM

-YE^iiT i:^UN

ANAi...Ys:n:i mode with modifications

iWEATI-iER DATA INPUT

JULIAN DATES PROCESSED: 9I,i89,8<CI9

>Y. >>: :>K >K >K jK >K %)}: >}J :>K >K :>}c :»}o}< :>}{•;}< jtc>K 5jo}{ :>K >K>ioK 3}c )}; >}c>}c>K >K 5}; >1<

:i. a ao 5 ;!.;l. .ao :l.;l. 6.90 6

,J70 . J70 ,-::|70 . '!:|70 .300 .850

CASK

tU'-JOWW j a . 50

3 . 0 0 3 . BO 0 .

0

3 . 60 a , BO 0 .

0

3 . 00 7 . -<:|0 0 .

0

3 . 60 7 . AO 0 .

0

S' fJ SOPS

•1

. 1 . a 60 5 60 a . 0

FENESTRATION

W INDOW

.1. .BOO

5.ao a. 80

:i. a 0.00 :i.;i. .ao a.io

BUILDING

•1 . 0 - ;i. . 0 -• :i. .0 7 . a 13 . a b . 9

. 6 0 0 . 6 0 0 . 6 0 0 . 60 0 .30 0

0 .

0

COROUNi;)

. aoo

0

ANALYS IS

3?i? 50 77 . 00 75 . 00 53

0 0 0 0 .1 . 1 1 1 1 .1 . 0 0

15

A 1 7.30

A 1 9 . 30

1 11.30

-^1 1 13.30

'^1 1 15 . 30

7 7.39

7 B 9 . 39

7 B 1 1 . 39

7 8 13 . 39

7 B ;l. 5 . 39

13 15 7 . ^:|6

13 15 9.J6

13 15 ll.-::i6

13 15 13. -^1

6

13 15 15. A6

CAI...CUI...ATE HOP

P35B

P35T

3
3. DESCRIPTION OF THE MEASUREMENTS

Interior daylight Illuminance was measured In a rectangular test room with a north-facing window. A schematic drawing of the test room is

shown in Figure 1. Illuminance on a horizontal plane vas measured in

the center of the room at a height of 76cm (2.5ft). The view out of

the window from this point was nearly unobstructed, except for a few

tree tops which obscured less than 10 percent of the sky view.

Illuminance was measured using silicon photovoltaic cells with cosine

diffusers and photometric filters. Concurrent measurements of solar

radiation and illumination levels were made for both diffuse (sky) and

global (sky and sun) components, to provide the input data necessary

for the CEL-1 simulations. Data were collected over a six-month

period to enable validation for a wide range of sky, solar and

seasonal conditions. A complete description of test room dimensions

and reflectances and window parameters is given in Table 2.

Table 2

Test Room Dimensions and Reflectances

Test Room

Width 1.6m (5.2 ft)

Height 2.1m (6.9 ft)

Depth 3.^m (11.2 ft)

Reflectances

Wall

Floor

Celling

0.1*7

0.30

0.85

Window - Double Pane

Width 1.6m (5.2 ft)

Height 0.9m (2.8 ft)

Visible Transmittance 0.80

4
4 . RESULTS

The validation results are presented in two formats. Figures 2 thru 6

each present a comparison of predicted versus measured illuminances

for each of five days. In these figures, calculated interior

illuminances are plotted for each sky type, and the calculated

illuminances for the sky types as chosen by CEL-1 are circled, and

labeled with the atmospheric extinction coefficient as determined from

measurement. Calculations were made for five times each day. Each

measured interior illuminance consisted of a one-hour average, so the

calculation used similarily averaged solar data. These five days

include clear, partly cloudy, and overcast sky types from different

times of the year. The days were chosen at random. From these

figures it is seen that the CEL-1 predictions tend to follow the

measured values closely, although some deviations are clearly evident.

Figure 7 presents ein overall comparison between the measurements and

predictions

A simple calculation of the standard deviation of the numbers formed

by taking the ratio's of the CEL-1 predicted values to the

corresponding measured values (or, alternatively, of the root-meansquare deviation of the ratio's from the constant model (unity) gives

a 2k% value. This can be thought of as representing a typical error of prediction. Expression in percent terms is preferred since the

width of the error band clearly must increase with the magnitude of

the prediction.

The levels of uncertainty described above are similar to the

uncertainties stated for other similar illuminance prediction

procedures, although validations of this type are rarely attempted.

Most of the uncertainty is due to the random nature of real skies,

which are rarely uniformly or predictably overcast or clear. Since

any one window sees only a small portion of the sky, small variations

in the luminance distribution of that portion of the sky can have a strong impact on interior illuminance, variations which would be too

subtle for a computer model to predict using horizontal solar

radiation as a driving function. This effect is illustrated in Figure

8, which presents a hemispherical photograph of a partly cloudy sky.

From this figure it is obvious that daylight illumination due to the

sky as seen through window A would be significantly greater than that

through window B, although both windows have nearly the same

orientation

Another uncertainty source is the determination of sky type. CEL-1

determines sky type on the basis of atmospheric extinction coefficient

which is calculated from the level of direct beam illuminance. Thus,

if no significant direct beam irradiance is incident, the sky is

assumed to be overcast, and high direct beam irradiances are assumed

to mean clear skies. These two assumptions are usually, but not

always, valid. A single cloud could completely extinguish the direct

beam solar radiation incident upon a surface, while the rest of the

sky remained clear, or the sky could be nearly overcast except in

front of the sun. Partly cloudy skies are particularly difficult to

predict, due to their wide dynamic range.

5
Taking into consideration the uncertainty associated with predicting

interior daylight levels, it is still of value to use such techniques

for evaluating fenestration performance. While one hundred percent

accuracy is not possible, it is possible to compare the relative

performance of various fenestration options as long as they are

compared on a fair and equal basis.

Thus, it is most important that the simulation be realistic and

reasonable even though the probabalistic aspects of sky modeling

cannot be completely addressed. The validation figures demonstrate

that the CEL-1 daylight illuminance predictions are reasonable for

typical office spaces and realistic for the purpose of comparing

fenestration options.

6
5. CONCLUSIONS

Comparisons of predicted and measured interior illuminance levels were performed using the CEL-1 Lighting Computer Program and measurements

made at the NBS Daylight Research Facility, Results of the validation

indicate a prediction uncertainty of +/- 24$, an uncertainty level which

is expected for this type of validation. The major sources of

uncertainty are believed to be associated with random and unpredictable

cloud patterns. It is concluded that the CEL-1 simulation procedure

would be of value in evaluating the daylighting performance of

fenestration systems.

7
6.

REFERENCES

\'

1. King, W.J., High, Performance Solar, Control. Offlce,tflndows.

Lawrence Berkeley Laboratory Report 7825 i Berekely, CA, 1977*

2. Bltterlce, M.G., McKinley, R.W., Use, Solar, Da_Yllght and

Heat from Windows to, Sa,ve, Fossir Fuel . PPG Industries, Inc.,

1978.

3. Owens, P.G.T., Eiiergy. S.pnggryAtJlgjQ^JuajLQfflgfi-.J^^

Pilklngton Bros. Ltd., Technical Advisory Service, 1976.

Kusuda, T., Gillette, G., Treado, S.J., Evaluation Of, The

P.ayJJ-gbtlhg- and-£nergy-f.firffirJBangfl. Kln.dPMg.».SKyIlgh,tg.i.and

Clerestories . National Bureau Of Standards NBSIR 83-2726,

June 1983.

5. Kusuda, T., Collins, B., Ana.lygAg-.of. IhenBia. and

LightInft Characteristics, of, Windows - Two Case Studies . National Bureau of Standards, Building Science Series 109»

February 1978.

6. Johnson, R., Selkowltz, S., Wlnkelman, F., Zenter, M., Glazing

Pp^JffllgatJigiLg.t»dy. for. En.er«y. EfflgJgngy-lJi gflHUBergial-gffJrgg

Buildings , Third International Symposium on Energy Conservation

In the Built Environment, Dublin, Ireland, March 1982.

7. Jurovlcs, S., Daylight., Glazing and Building, Energy, Minimization.

IBM Scientific Center publication, Los Angeles, CA., 1981.

8. Place, W., Fontoynant, M., Bauman, F., Anderson, B., Howard, T.,

Commercial Building, DavUghtlng . LBLO-14348, Lawerence

Berkeley Laboratory, Berkeley CA, 94720, 1982.

9. CEL-1 Lighting Computer Program - Programmer's Guide. CR83*009»

Naval Civil Engineering Laboratory, Port Hueneme, CA, 93043»

January 1983.

10.

CEL-1 Lighting Computer Program - User»s, Guide . CR81.026,

Naval Civil Engineering Laboratory, Port Hueneme, CA, 93043

September 198I.

8
Figure 1 , Layout of Test Room

9
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16

!
A I RE V. 2-«C)

^.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 84-2937

2. Performing Organ, Report No , 3. Publication Dace

j

4. TITLE AND SUBTITLE

j Validation of Daylight Prediction With CEL-1

5. AUTHOR(S)

Steve Treado, Cindy Francisco and Douglas Holland

6. PEI’.I^ORMING ORGANIZATION (If joint or other than N BS. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

see instructions) 7. Contract/Grant No.

8. Type of Report & Period Cc\e

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State. ZIP)

10. SUPPLEMENTARY NOTES

I

i

I

I

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Calculations of interior illuminance levels using the CEL-1 computer program are compared to measurements, for a typical office space. The comparisons are made for

a wide range of sky conditions, solar intensities and seasonal intervals using a north facing window. The statistical uncertainty associated with the interior

daylight calculations is examined and the sources of the uncertainty are discussed.

The results show that for all sky types, the interior illuminance calculations have

an uncertainty of +/-24%, with the major uncertainties being due to unique and

unpredictable sky luminance distributions and difficulty in determining the

appropriate sky condition from standard weather data.

12. KEY WORDS fS/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semcolors

building energy, daylight, illuminance, fenestration, solar radiation , window

13. AVAILABILITY

14. NO. OF

PRINTED PACES

1 X 1 Unlimited

1 1 For Official Distribution. Do Not Release to NTIS 2^

20402. 15. Pm —

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1 Order From National Technical Information Service (NTIS), Springfield, VA. 22161

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